1. Field of the Invention
This invention relates to a sling assembly for lifting a pack of loose sheets, e.g., glass sheets.
2. Discussion of the Prior Art and Technical Problems
Glass sheets are shipped from a glassmaking plant to a fabrication plant by truck or railcar. In general, packs of loose glass sheets are loaded in a generally vertical position supported on deck runners against a back supporting frame. The packs are separated from one another and from the supporting frame by vertical runners to facilitate removal of the packs by a sling assembly. Normally the packs are tilted 5.degree. from the vertical plane for packing stability.
The prior art sling assemblies, e.g., as taught in U.S. Pat. No. 3,838,779 include a spreader bar having a carrying yoke adapted to be supported by a hoist. A pair of slings made of polyester webbing are adapted to be looped over the spreader bar at their upper ends and looped under the glass packs at their lower ends. The bottoms of slings are provided with protective plates (1) made from steel to prevent the glass edges from cutting the sling material and (2) coated with rubber to prevent damage to the glass edges. At the midpoint of each sling is secured a horizontal strap that secures the edges of the glass pack to prevent racking and subsequent slippage.
Although the sling assembly taught in the above-mentioned U.S. Pat. is acceptable for lifting glass packs, there are limitations.
As previously mentioned, the glass packs are separated from one another by vertical runners. These vertical runners limit the insert distance of the slings looped under the sides of the pack.
The distance of the vertical runners from adjacent sides of the pack is selected to prevent bending forces from acting on the cantilever sides of the glass pack during transit. However, this distance is not the same to minimize bending forces so as not to excessively bow the glass pack when handled by the sling assembly.
For example, packs of 5 glass plates each about 130 inches (3.3 meters) by 180 inches (4.6 meters) and 1/8 inch (0.32 centimeters) thick have vertical runners spaced about 10-16 inches (25.4-40.6 centimeters) from an adjacent side of the pack. Therefore, the straps of the sling assembly engage the bottom edge of the pack for lifting at a position spaced about 8-14 inches (20.3-35.6 centimeters) from adjacent side of the glass pack.
When the pack is handled by the sling assembly, the pack excessively bows in the center because of excessive unsupported distance between the straps.
The glass pack may be made stiffer by increasing the number of plates, but the packs become uneconomical and unpractical to handle because of the increased weight of the pack, e.g., from 4,000 pounds for 5 glass plates to 10,000 pounds of 12 glass plates. In other words, a hoist or crane of increased weight lifting capacity has to be used.
Another limitation of the prior art sling assemblies is the protective plate that protects the straps against the edges of the glass sheets. The glass packs, as mentioned, are tilted toward the support member for stacking stability. When lifting a glass pack from a tilted position to a vertical or upright position, the straps equalize their length. Normally the protective plates are secured to the strap, therefore movement of the strap tends to shift the protective plate. This shifting of the protective plate can damage the edges of the glass plates.
It would be advantageous therefore to provide a sling assembly that does not have the limitations of the prior art.